(1) Field of the Invention
The present invention relates to a process and apparatus for removing ink from wastepaper. More particularly, the present invention relates to a process and apparatus for removing ink from wastepaper at a high ink-removing efficiency to obtain a high yield of paper pulp having an enhanced brightness.
(2) Description of the Related Art
It is well known to remove ink from wastepaper by preparing an aqueous slurry of wastepaper pulp treated with a chemical, for example, a carbon-removing agent or bleaching agent, in a treating vessel, by blowing air from the bottom portion of the vessel into the wastepaper pulp slurry in the treating vessel so as to cause the resultant air bubbles to absorb ink particles floating in the wastepaper pulp slurry while the bubbles rise to the surface of the wastepaper pulp slurry, and then by removing the resultant ink-absorbed froth from the wastepaper pulp slurry.
In the above-mentioned ink-removing process, to enhance the ink-removing efficiency, it is important to cause a uniform flow of a number of fine air bubbles into the wastepaper pulp slurry, to enhance the chances of contact of the ink particles with the fine air bubbles.
For this purpose, various approaches have been made. For example, U.S. Pat. Nos. 4,186,094 and 4,347,128 disclose one such approach in which an eddy stream of a wastepaper pulp slurry is formed in a treating vessel, an additional amount of the wastepaper pulp slurry is supplied into the vessel at angles tangential to the eddy stream so as to flow the wastepaper pulp slurry in the eddy stream, and air bubbles are blown into the eddy stream of the wastepaper pulp slurry. However, the ink-removing effect of the above-mentioned method is unsatisfactory.
Japanese Unexamined Pat. Publication No. 50-25804 published on March 18, 1975 for Escher Wyss GbmH discloses another approach in which air is blown through a number of holes perforated on a plate fixed in the bottom portion of a treating vessel.
DE-OS No. 2,409,235 published on Oct. 23, 1975, for Escher Wyss GmbH, discloses still another approach in which air was blown through a number of holes formed on a periphery of an air-blowing pipe fixed in the bottom portion of a treating vessel.
However, the holes on the plate are sometimes blocked by dusts in the wastepaper pulp slurry and the resultant ink-removing effect is unsatisfactory.
It is generally believed that the brightness of the ink-removed wastepaper pulp is increased with an increase in the ratio (G/L) of the entire volume (G) of the air bubbles in the treating vessel to the volume (L) of the wastepaper pulp slurry in the treating vessel. However, it is also believed that the increase in the brightness of the ink-removed wastepaper pulp reaches saturation, at a certain point, even if the ratio (G/L) is further increased. That is, an excessive increase of the entire volume (G) of the air bubbles is not effective for enhancing the brightness of the resultant pulp and is not economical. Therefore, in the conventional ink-removing process, the ratio (G/L) is limited to a relatively small value, that is, 1.5 or more but less than 5.0.
Furthermore, in the conventional ink-removing processes and apparatuses, it is taught that it is very difficult and uneconomical to increase the ratio G/L to 5.0 or more.
In the other approach, the wastepaper pulp slurry which has been treated with the air bubbles is further washed one or more times with fresh water. This washing procedure results in a decrease in the yield of the resultant ink-removed pulp and also in the contamination of environmental water by the waste water discharged from the washing procedure.
Under the above-mentioned circumstances, it is desired by the industry that a new process and apparatus be provided that will effectively remove ink at a high efficiency and with a high yield of ink-removed pulp having an enhanced brightness.